Electromechanical properties of langasite resonators at elevated temperatures

Langasite is a single crystalline material with potential applications as piezoelectric transducer at elevated temperatures. Anticipated applications include resonant gas sensors with the ability to detect the gas composition in, e.g. SOFCs or their gas reformers. The material exceeds significantly the operation temperature limit of quartz and exhibits bulk oscillations at temperatures of at least 1400 °C. Commonly used Y-cut shear-mode resonators show a strong dependence of the resonance frequency on the operation temperature. As long as variable operation temperatures have to be encountered, a frequency compensation using the third overtone of the resonator or other elaborated compensation methods have to be applied. The data acquisition can be simplified for fixed operation temperatures occurring often during routine measurements. Under such circumstances, the application of temperature compensated crystal cuts is favourable. This paper focuses on the determination and discussion of all components of elastic and piezoelectric tensors of langasite at elevated temperatures. The resonance spectra of several langasite samples have been measured and fitted with the impedance calculated from a one-dimensional physical models of piezoelectric bodies vibrating in several modes. In order to extract the electromechanical parameters, different resonator geometries and orientations are used. These properties are determined at temperatures up to 900 °C. Based on this data, a crystal cut with minimised temperature dependence is predicted.